c# - LINQ to Entities 中的 Enum.HasFlag？

Question

我有一面旗帜，例如

[Flags]
public enum DaysOfTheWeek
{
    Monday = 1,
    Tuesday = 2,
    Wednesday = 4,
    Thursday = 8,
    Friday = 16,
    Saturday = 32,
    Sunday = 64
}

如果我想使用 Linq 基于包含特定标志的变量进行过滤，我可以尝试使用 Enum.HasFlag在 lambda 语句中过滤多个标志，例如

DaysOfWeek weekendFilter = DaysOfTheWeek.Saturday | DaysOfTheWeek.Sunday;
var weekends = allDays.Where(d => d.DayOfWeek.HasFlag(weekendFilter));

目前给出:

LINQ to Entities does not recognize the method 'Boolean HasFlag(System.Enum)' method, and this method cannot be translated into a store expression.

Answer 1

枚举标志基于二元运算。

HasFlag = (allDays & (DaysOfTheWeek.Saturday | DaysOfTheWeek.Sunday)) != 0;

如果您愿意，您可以将其抽象为扩展方法。

具体回答你的问题:

DaysOfWeek weekendFilter = DaysOfTheWeek.Saturday | DaysOfTheWeek.Sunday;
var weekends = allDays.Where(d => (d.DayOfWeek & weekendFilter) != 0);

我知道您可以将此谓词与 LINQ to SQL 结合使用。它将被翻译成 SQL(SQL Server 很好地支持二进制算法)。不确定 EF 是否同样有能力，因为它的 LINQ 支持通常远不如 L2S。